1. Field of the Invention
This invention relates to drill strings employed in well drilling, and more particularity to a method and apparatus for drilling straight wells by preventing unwanted spiraling progressions and slotting effects generally associated with drilling through relatively irregular formations, specifically through extra hard earthen formations.
2. Background of the Invention
Drill string collars generally used in drilling operations consist of long columns of thick walled tubes directly above the drill bit. These collars add additional weight to the drill string pipe to push the rotating drill bit through earthen formations. These drill string collars are generally connected to the thousands of feet of drill string pipe connected thereabove. A rotary drilling rig turns the drill string pipe, which turns the drill collars, which turn the drill bits used in creating a well.
It is well known in the drilling industry that when a drill bit comes into contact with an extra hard earthen formation, especially those formations positioned at an acute angle, the drill bits tend to drift, thus making an elongated hole or slot through the extra hard earthen formation. This drift is caused by the drill bit's attempt to take the path of least resistance as it creates a borehole. Since the drill string pipe which spins to drive the drill bit is not as large in diameter as the drill bit, the slot tends to be larger down-slope than at the first contacted up-slope area of the formation. Thus, when the bit contacts an extra hard earthen formation at an acute angel and drifts, an elongated hole or slot is created which has a much smaller diameter on the up-slope of the formation. This elongated hole or slot causes problems when attempting to withdraw the bit or when running other tools into the well. This dilemma is known in the industry as “key slotting” or “key holing.” Once the drill bit has completely penetrated this extra hard formation, the drill bit tends to follow a course consistent with the exiting course of the drill bit as it exits the formation.
In any angled or deviated borehole, there exists a force generated by the pendulum effect of the lower end of the drill string. The earth's own gravitational force exerts a downward pull upon the drill string whereby the drill string reacts to this pull by trying to swing through the lower side of the hole toward a true vertical orientation. The use of heavy drill collars did not have enough force generated by the pendulum effect to overcome the physical and structural forces of the earth's strata which causes the drill bit to deviate. In other words, the force tending to deviate the drill bit is greater than the counter-force tending to return the drill bit to vertical.
Another problem encountered while drilling through extra hard earthen formations is based upon the application of applying weight to a rotating drill string having torque applied thereto. The application of weight forces the drill bit against the formation as it is rotated by the long drive shaft action of the drill string pipe. The drill string pipe is rotatably driven to provide the rotary drilling action of the drill bit. The rotation of the drill string itself, coupled with the biting effect of the rotary drill bit as it contacts a formation as well as the applied weight and torque to the drill bit, tends to bow the drill string pipe and causes the drill bit to take a spiral-like path often referred to as the “corkscrew effect.” This corkscrewing of the drill pipe may be quite pronounced in some cases having a spiral several feet in diameter and up to three complete spirals per 100 feet of well depth. Obviously, such spiraling uses more pipe footage and requires more time to drill than would be required with drilling straighter bored wells.
Various types of stabilizers and friction-reducing technologies have been employed to reduce this corkscrew effect, eliminate vibrations and the key slotting problem. Some drilling operators use an adjustable, rotatable sleeve surrounding the main body of the tool which allows the joint to be adjusted to compensate for out of balance conditions. Other stabilizing tools have blades which may be mechanically or hydraulically positioned outwardly relative to the tool body to provide counter balance to the rotating string. In any case, the prior methods' objects are the same, to reduce the amount of wobble, imbalance or vibration in the drill string in an attempt to straighten the drill paths through earthen formations.
A wholly opposite approach has also been applied in the present invention to solve both the key slotting and corkscrewing complications described above. In the 1960's, Cyril Hinds, the inventor of the present invention, modified standard 30 foot to 40 foot long tubulars by boring into one half of the outside surface of the tubular's walls. Essentially, bores were made along one side of the tubulars which created cavities within one half of the surface area of the drill collar. In operation, these tubulars were thought to create an unbalanced condition in the drill string which would compensate for the natural tendency of the drill bit to walk or drift away from the intended drill path. However, these modified tubulars tended to fracture due to the moment created as a result of the unbalanced rotation of the drill string coupled with the weakened tubular wall due to the borings. Thus, the modified tubulars would break off within the well and had to be fished out of the wells creating severe delays and additional expenses.
U.S. Pat. No. 3,391,749 issued to Arnold shows a drill collar which is eccentrically weighted with respect to its longitudinal axis of rotation to prevent deviation from its vertical path by drilling blind holes along a side of the collar. These drill collars tended to break under the stress generated by the high torque pendulum effect.
U.S. Pat. No. 4,068,730 issued to Arnold shows an improved drill collar which is eccentrically weighted to its longitudinal axis of rotation to prevent deviation from its vertical path by drilling blind holes along the side of the collar that vary in size in a cyclical pattern along the length of the collar. These drill collars sought to overcome the weakened conditions associated with U.S. Pat. No. 3,391,749.
U.S. Pat. No. 4,190,122 issued to Arnold shows numerous drill collars which are eccentrically weighted to their respective longitudinal axis of rotation to prevent deviation from its vertical path by drilling blind holes along the side of each collar to prevent the drill collar string's rubbing contact with the sides of the borehole.
U.S. Pat. Nos. 3,391,749; 4,068,730; and 4,190,122 have a common design flaw in common. The blind holes, grooves, slots, etc. disposed on the outer surface of the drill collar as disclosed in each of these patents tend to fill-up with drilling fluids, mud and debris during drilling operations. The material filling-up the various cavities formed on the surface of the drill collar will reduce and/or eliminate the eccentric weight these patents seek to achieve. Therefore, these devices must be continually washed and unclogged to keep their desired eccentric weight to be effective.
U.S. Pat. No. 4,776,436 issued to Nenkov et al. shows a drill collar with an internal 360 degree cavity formed withing the walls of the drill collar filled with articles to dampen and/or absorb shock. Nenkov et al. shows a uniform dispersion of the articles filling the 360 degree cavity with absolutely no off-balance.
U.S. Pat. No. 4,522,271 issued to Bodine et al. shows a similar drill collar with a 360 degree cavity filled with balls, pellets and mud to dampen sonic waves and absorb shock. Bodine et al. also shows a uniform dispersion of the articles filling the 360 degree cavity with absolutely no off-balance.
U.S. patent application Ser. No. 08/999,620, filed by Dewey E. Owens on Mar. 24, 1997, ABANDONED, disclosed a drill collar including a 180 degree cavity within the cross sectional area of the drill collar's walls including a magnetic strip therein and a plurality of steel balls contained within the cavity to provide a counter balancing effect of the drill string pipe while driving a bore hole, whereby the magnetic strip attracts the steel balls to stabilize the drill string and reduce wobble created by any imbalance created by the 180 degree cavity. This invention described in U.S. patent application Ser. No. 08/999,620 was additionally offered for sale on Apr. 16, 1998. The purpose of the invention described in U.S. patent application Ser. No. 08/999,620 was to eliminate the wobble and/or oscillations experienced by drill collars by a counter balancing effect.
The objects, features and advantages of the present invention will become apparent from the drawings and descriptions given herein, and the appended claims.